Chemistry Reference
In-Depth Information
Et
CN
O
Et
Et
CO
P
NC
CN
Br
Br
II
V
Ni
Re
Br
CO
III
0
OC
Fe
Br
Ni
O
NC
CN
Br
Br
CO
Br
NCS
SCN
VI
CO
P
V
Et
Et
NCS
SCN
Et
NC-CH
3
Cl
NO
Et
NCH-CH
3
Cl
Et
I
II
S
-
S
H
3
C-CN
Co
Cl
Cu
II
Fe
N
C
C
N
NC-CH
3
Cl
S
-
S
NC-CH
3
Et
Cl
Et
square pyramidal
trigonal bipyramidal
Figure 4.16
Examples of complexes adopting one of the two shapes of five-coordination.
metal lies typically between 30 and 50 pm above the square plane of donors, close to the
value of 48 pm calculated from geometry assuming an apical donor-metal-basal donor
angle of 104
◦
for 200 pm metal-donor bonds. In this stereochemistry, the single axial bond
tends to be longer than the four equatorial bonds; for [Ni(CN)
5
]
3
−
, the former is 217 pm
and the latter are 187 pm.
4.2.6
Six Coordination (ML
6
)
ML
6
is the most common coordination type by far that is met for transition metal elements
(seen for all configurations from d
0
to d
10
), and also is often met for complexes of metal
ions from s and p blocks of the Periodic Table. Of the two limiting shapes (Figure 4.17),
the
octahedral
geometry is by far the most common, though a few examples of the other
limiting shape,
trigonal prismatic
, exist. Because the six donor atoms come into closer
contact in the trigonal prismatic than in the octahedral geometry, trigonal prismatic is
predicted to be less stable. However, many structures show distortion that places them as
intermediate between ideal octahedral and the ideal trigonal prismatic form. This distortion
is best viewed in terms of the orientations of two opposite triangular faces of sets of three
donors. For octahedral (which is also a special case of trigonal antiprismatic, where edge
and face length uniformity applies), the 'top' face is twisted around 60
◦
versus the 'bottom'
face; for trigonal prismatic, the two faces exactly superimpose (the twist angle has been
reduced to 0
◦
). Intermediate or distorted structures show a twist angle of between 60
◦
and
0
◦
(Figure 4.17).
The trigonal prismatic geometry is usually enforced by the ligand. Simple ligands almost
exclusively form octahedral complexes, as do the vast majority of polydentate ligands. Of
complexes of monodentate ligands, [Re(CH
3
)
6
], [Zr(CH
3
)
6
]
2
−
and [Hf(CH
3
)
6
]
2
−
(which
are d
0
-bonded ligands that display
trigonal prismatic geometry. A fairly rigid chelate with a short separation between the two
donor atoms (the so-called 'bite' of the ligand, the preferred separation of donors from each
or d
1
systems) are three of a very few with simple
